The present invention relates to a method for treating metal containing waste water while employing a vessel containing the waste water in which at least one anode and one cathode are disposed, the cathode consisting essentially of electrically conductive particles and in which the waste water is subjected to electrolysis during which the waste water and the cathode particles are being moved and to an apparatus for practicing the method. Such a method is disclosed in Australian Pat. No. 46,691.
Waste waters in the sense of the present invention are understood to mean all metal containing solutions obtained during technical processing. Such solutions are, for example, waste waters from mines, final liquors, wash waters, trickling waters, rinse waters from tanneries, electroplating operations, the manufacture of printed circuits, film developing, etc. Electrolytic processes have been in use for a long time for the recovery of metals from such solutions having a relatively high metal content, so-called concentrates and semiconcentrates. In this way, it is possible to directly cathodically separate a major portion of the metal content of such solutions. If the electrolysis is effected in baths having vertical electrodes, final concentrations of about 8 to 1 g/l are attained, depending on the electrochemical properties of the metals to be separated, the bath composition and the content of additives. However, with such metal contents of 8 to 1 g/l, the current yield is already greatly reduced and further reduction in the metal concentration of the solutions is practically unattainable with this mode of operation. The then remaining solutions cannot be economically processed with the aid of ion exchangers, since the requirement for regenerating chemicals for the ion exchanger would be unduly high and the resulting heavy salting of the waste water would constitute an additional stress on the environment.
In order to further reduce the residual metal content in the solution, it has often been proposed to improve the cathodic deposition conditions. Such proposals are essentially directed to reducing the depletion of the electrolyte of depositable metal ions in the region of the cathode. Thus, various types of electrolytic cells are known which contain spatially fixed, firmly contacted, quasi two-dimensional electrodes which are frequently arranged vertically, but which can also have another orientation. A relative movement of the electrodes with respect to the electrolyte serves to improve deposition conditions.
Movement of the electrodes can be realized by vibration. The electrodes may be designed to have a disc, ring or cylindrical shape, and may be rotated in the electrolyte. Further, the exterior configuration of the electrodes may contribute to the fact that the electrolyte flows against the electrode surface at high speed and, if possible, not merely in a laminar manner. The electrolyte may also be pumped through channel type electrolysis cells, may flow vertically through concentrically arranged electrodes with gases being blown in simultaneously. The vertical movement of the liquid may be so intensive that glass beads or other particles of various materials are stirred up and improve the mass transfer at the vertical electrodes.
In order to realize a higher space-time yield during electrolytic processes, numerous efforts have been directed at building up three-dimensional electrodes from electrically conductive bulk material. Such particle piles are called fixed beds if the relative movement with respect to the electrolyte is effected by passing the electrolyte through the bed, either in the direction of the electric field lines or normally thereto.
It is also known to improve conditions for deposition of metals from concentrates and semiconcentrates by not only moving the electrolyte relative to the bulk of the cathode, but simultaneously moving the bed itself. According to East German Pat. No. 114,624, the pile of particles is moved by disposing the particles in a container having a perforated bottom, and rotating the container about an axis which is inclined with respect to the horizontal. During the rotation, the electrolyte flows through the pile. In such a method, the anode can be disposed not only above, but also below the cathode pile. As has been found in practice, this process is suited only for processing concentrates and semiconcentrates and has the above-described drawbacks.
Australian Pat. No. 46,691 discloses a process as described above. At least one stirrer is provided to move the cathode pile. The stirrer mechanically moves the particle pile at the bottom of the container through which the electrolyte flows. In this process, the anode may also be constituted by electrically conductive particles and is disposed above the cathode pile. Although the pile cathode in this known process is agitated, under simultaneous movement of the electrolyte, so that the particles are continuously and circulatingly moved, this process is possible only for treating concentrates and semiconcentrates and it has so far not been possible in practice to reduce the metal concentration of the waste water to a residual content of about 1 g/l because the pile cathode has dead spaces and potential free zones in which the electrolytically deposited metal is chemically redissolved in part.
All of the above-described processes and apparatuses are suitable for the electrolytic processing of concentrates and semiconcentrates, particularly if these substances are returned to the process after the electrolytic treatment because under such conditions it is not necessary to realize essential demetallization. The above-described cells with a moving cathodic particle fill can be used only to deposit electropositive metals such as silver, copper and bismuth. This is so, inter alia, because of the low surface-volume ratio of the cathodic particle fill and because of larger field-free interiors in the same where electronegative metals are redissolved by the electrolyte.
In all of the known processes for the electrolytic recovery of metal from concentrates and semiconcentrates, at sufficiently high current efficiencies the end result is a solution with a metal content around 1 g/l. If these metals cannot be returned to the process, the remainder of the metal ions is precipitated in a conventional manner by raising the pH and the slurry is dewatered and the metal containing residue is discharged.
Attempts have also been made to process such diluted rinse waters directly electrolytically. If a cathodic particle pile is used for this purpose and the waste water is passed through this pile from the bottom of the cell vertically upwardly at such a high speed that a turbulent (fluidized) bed is produced, this brings about a large cathodic surface and the conditions for a cathodic deposition of metal ions from solutions having concentrations of less than 1 g/l are much improved. In small experimental diaphragm containing cells, such a mode of operation has yielded residual content of copper and silver in the solution of about 1 mg/l. The technical use of a fluidized bed electrolysis is made difficult, however, by apparatus problems. The low conductivity of diluted waste waters in conjunction with a diaphragm leads to a high voltage requirement. Further, the diaphragm easily becomes clogged, for example, by metal deposited thereon. A change in particle size also results in a change of the kinematics of the fluidized particles and contacting the fluidized bed is fraught with problems.
The electrolytic recovery of metals from concentrates and semiconcentrates thus is considered state of the art "while the direct electrolytic processing of diluted rinse waters is out of the question." See, R. Weiner: Die Abwasser der Gavanotechnik und Metallindustrie [in translation, The Waste Waters of the Electroplating Art and Metal Industry] 4th Edition, 1973, page 210, published by Eugen G. Leuze Verlag, Saulgau. For this reason, diluted waste waters today are not treated electrolytically, but chemically, either directly or after enrichment. After decontamination, the waste water is subjected to neutralization so that the majority of the metal ions are precipitated out. The waste water is then clarified and drained in a sewer. The remaining mud (sludge) is thickened, dewatered and discharged. Such process is economically unjustifiable and untenable in the long run because of the effect on the environment.